Gas reacting apparatus and method

ABSTRACT

Gas reacting apparatus and method are described for wet mass transfer of solute gases from a gas stream with a liquid or slurry reacting medium capable of chemisorption of solute gases in the gas stream. The apparatus comprises an elongated conduit defining a primary reaction zone in fluid-flow communication with a fan defining both a secondary reaction zone and a spray coalescence zone, and a plurality of dual-fluid spray nozzles coaxially spaced in series within the conduit and countercurrently or cocurrently directed to the gas stream for spraying the liquid or slurry reacting medium into said conduit to form a plurality of spray contact zones of uniformly-distributed fine droplets wherein intimate contact of high interfacial surface area between the sprayed liquid or slurry and the gas stream is effected to remove solute gases from the gas stream. The fan is operated at a relatively low speed to promote coalescence of droplets through droplet-droplet collisions followed by collection on the fan blades with liquid or slurry layers which, under the action of centrifugal forces, are separated in the form of an attendant annular spray zone of relatively large droplets to trap more of the solute gases. The droplets upon impingement on the fan casing are collected and removed by gravity, while the clean gas stream is exhausted from the fan casing. The apparatus may also include a quenching structure for saturating and cooling the gas stream prior to the removal of solute gases as well as an entrainment separator for further removing entrained droplets not separated by the fan from the clean gas stream.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 020,953 filed Mar. 2, 1987, now U.S. Pat. No.4,865,817.

FIELD OF INVENTION

This invention relates to gas/liquid, gas/liquid/solid andgas/gas/liquid mass transfer, and more particularly to amechanically-aided gas reacting apparatus and method for masstransferring solute gases from an industrial or utility gas stream intoa liquid or slurry reacting medium, if necessary in conjunction withsimultaneous particulate matter removal, wherein the mass transfer maybe a purely physical phenomenon or may involve solution of the solutegas in a liquid or a slurry suspension, followed by chemical reactionwith one or more constituents in the liquid or slurry reacting medium.This invention also relates to a gas reacting apparatus for effectinggas-gas reactions in fluid flow communication with an integrated wetseparation of the resultant reaction products which are in the form of afinely divided particulate matter. While not limited thereto, theinvention is particularly well suited for the removal of SO₂ and othergaseous pollutants from waste gas streams such as those emanating fromelectrical utilities, smelters and industrial boilers.

BACKGROUND OF THE INVENTION

For maximum efficiency, it is desirable to make a gas reacting apparatuswherein a high interfacial surface area coupled with turbulent mixingand long residence time are effected simultaneously and which, as such,is capable of removing both solute gases and particulate matter, eitherseparately or simultaneously with high efficiency, with the former beingseparated via a gas-liquid, gas-liquid-solid or gas-gas-liquid reaction,depending on the application. To-date, as far as we are aware, there iscurrently no one type of gas reacting apparatus available that iscapable of achieving high performance for all of the above criteria, dueto a compromise generally being made between generation of very finedroplets for affecting very high surface area on one hand and longresidence time on the other.

One of the methods for separating particulate matter in solid or slurryform from a gas stream wherein a dirty gas stream enters a conduit atone end and is moved through it by a fan at the other end and where afine spray of liquid, preferably water, is concurrently discharged intosuch a gas stream upstream from the fan is described in U.S. Pat. No.4,067,703, issued Jan. 10, 1978, the disclosure of which is incorporatedherein by reference. The patent disclosure, while showing ahighly-effective method for removal of particulate matter from a gasstream, does not teach how the apparatus can be used as a gas reactingapparatus for removing solute gases. Also, in many aspects, thetechnique disclosed therein does not provide the absorbing and reactingenvironment required for effecting high removal efficiency of solutegases. For example, in the foregoing prior art patent, the mixture ofgas and particulate matter enters only a single contact spray zoneprovided by one nozzle in which an atomized liquid spray is injectedcocurrent to the dusty air stream flow. While this mode of operation asdisclosed has proven to be highly effective for removing particulatematter and effecting lower pressure drop in the apparatus whereinparticles were collected primarily by impaction upon the finely-dividedwater droplets introduced, followed by further agglomeration andimpaction on the fan blades as the gas moves through the device, theresidence time available for mass transfer is too short and theeffective interfacial surface area and turbulence generated by a singlecontact spray cocurrently oriented to the gas stream are not sufficientto effect high removal efficiencies of solute gases of relatively lowsolubility in aqueous solution. It is, therefore, desirable to providefor an improved gas reacting apparatus and method which overcome some ofthe shortcomings of the foregoing prior art apparatus in which increasedavailable residence time, interfacial surface area and turbulence aregenerated to result in accelerated absorption and reaction kinetics andintimate gas/liquid contact and thus, in high removal efficiency of bothsolute gas and particulate matter.

While high interfacial surface area, turbulent mixing and long residencetime for effecting accelerated mass transfer of solute gases andeffectively separating particulate matter are the major criteria in gasreacting apparatus selection, often a compromise must be made betweenremoval efficiency on one hand and operating reliability on the other.Several other factors then also enter into consideration, such as slurryhandling without plugging, turndown, and gas and liquid distribution.

The basic processes for removal of solute gases from gas streams,particularly flue gas desulfurization processes, are based onreadily-available, low-cost absorbents in the form of an aqueous slurry,such as a lime or limestone slurry, or a clear aqueous solution, such ascaustic or ammoniacal solutions. Various prior art methods are in use tobring the above absorbing and reacting media into intimate contact withthe pollutant-laden gas. Packed bed and perforated trays, which areknown to be efficient gas absorption and reaction devices, are usuallythe first choice for designers of flue gas desulfurization (FGD)systems, but experience has shown that they are not completelysatisfactory. Both perforated trays which bubble the gas through a thinlayer of liquid, and packed beds, which pass the gas over solid packingelements that are wetted with the liquid have many narrow passages whichare subject to plugging especially if particulate loads are heavy, or ifprecipitates are formed during the chemisorption process. Suchconditions can be minimized by careful process design, but thepossibility of scaling under upset conditions still exists andcompromises reliability. Another principal disadvantage of both of theabove types of scrubbers is their extremely limited turndown capability.

Consequently, heretofore, the gas reacting devices of preference and theones that would seem to be the answer have been the venturi or openspray tower wherein the internal complexity is low and yet where arelatively large surface area of the liquid is generated per unit volumeof gas treated While the above devices have evolved considerably overthe last decade in a way to improve their performance and to remove someof their shortcomings, the current trend in the design particularly ofFGD systems, is away from venturis to spray towers or combinationtowers. The venturi design, although capable of producing a relativelylarge liquid surface area for contact with the gas stream, was abandonedlargely because the very short liquid/gas contact time (attributablelargely to the absorbing medium being introduced cocurrently to the gasstream in the throat of the venturi) results in low sulfur dioxideremoval. Also, being a relatively high energy device, it is incapable ofproducing an evenly distributed regime of droplets at high densityunless an `overkill` situation exists wherein excess energy in the formof velocity pressure is added to the gas stream to provide for therequired uniform distribution. Spray towers, on the other hand, have fewinternal components in the gas/liquid contact zone and the use of spraysappears to offer an easy way of increasing the surface area exposed tothe gas. However, the sprays are usually introduced at the top of thespray tower and drop by gravity in counter-current flow to the gasstream. To avoid being entrained in the gas stream, the normal size ofthe droplets sprayed is in the order of 1000 to 2500 microns indiameter. Thus, to increase the surface area exposed to the gas phaseand residence time, very high liquid to gas (L/G) ratio and large towersmust be employed, all of which substantially increase the capital andannual cost requirement. To effect good gas distribution, a large numberof spray nozzles must be used, so that the tower cross-section isuniformly covered with the spray pattern. However, failure of one or twonozzles usually creates a path of least resistance through which the gascan flow, thereby reducing the efficiency of the apparatus.

In addition, the large size of droplets used in spray towers reducessubstantially the capability of the apparatus to efficiently remove dustparticles in the low particle size range, typically less than 3 microns.With the larger droplets, the decreased gas-liquid surface area can becompensated for by increasing the tower size, the number of sprayheaders, and circulation rates of the scrubbing liquor, all of whichincrease the tower space requirement, thereby initial cost and energyconsumption. Droplet entrainment and mist elimination, while rathereffectively being addressed by the production of larger droplets, canstill be the "Achilles heel" of spray tower operation, because it is theonly part of the operation where gas flow must be somewhat restricted.These limitations and the fact that the spray and venturi apparatus eachoffers advantages not shared by both, have given rise to the developmentof combination gas reacting devices. These combination arrangementsgenerally combine the features of venturi and spray apparatus into onemodule. These recent designs offer greater performance, allowing highremoval efficiency of both gaseous pollutants such as SO₂ andparticulate matter such as fly ash, but at a very high cost. It is,therefore, desirable to provide an improved gas reacting apparatus whichcombines all of the advantages offered by venturis and spray towers intoone apparatus.

SUMMARY OF THE INVENTION

The problems and disadvantages associated with prior art systems areovercome by the present invention by providing a gas-reacting apparatusand a method which is simple, economical and capable not only ofproviding good turndown and gas-liquid distribution, but also capable,on the one hand, of generating high turbulence and many fine droplets ofan aggregate surface area many times larger than produced in a spraytower of considerably larger size and, on the other hand, of providingfor a longer available residence time and higher surface area than in aventuri, thereby effecting high removal efficiency of both solute gasesand fine particulate matter and yet operating the apparatus withsubstantially decreased amounts of liquid, low energy and spacerequirements. It has been shown that the amount of liquid used by theimproved gas reacting apparatus is only about 10% of that required by asuitable spray tower and only about 2% of that required by the venturiwith comparable efficiency.

According to the invention, a gas stream containing solute gases or bothsolute gases and particulate matter is passed through a conduit andcontacted while flowing through the conduit by at least two sprays ofliquid or slurry, preferably injected countercurrent to the gas stream.

In the conduit, the liquid or slurry absorbing-reacting medium is finelyatomized by nonplugging, dual-fluid nozzles, which are preferablycentrally disposed and spaced in series in the conduit to form two ormore contact spray zones, and adapted to spray droplets in the rangeabout 5 to about 100 microns, more usually about 5 to 30 microns. Byspraying such liquids or slurries into a suitable reaction chamber, atremendous number of droplets are generated along with very high surfacearea. For example, if only 5 micron droplets are generated, eachkilogram of water yields about 1.5×10¹³ droplets which have a surfacearea of about 1200 square meters. On the other hand, in a traditionalsystem, if only 1000 micron droplets are generated, each kilogram ofwater yields about 1.9×10⁶ droplet which have a surface area of about 6square meters. These surface area figures as shown above are by an orderof magnitude greater than generated by the commercially-availabledevices presently used for this service. Since the mass transfer that agiven dispersion can produce is often proportional to (1/D)², finedroplets are greatly favoured.

Upon intimate contact of the solute gas and particulate matter with thesprayed absorbing-reacting medium, transfer of the solute gas andparticulate matter from the gas stream to the absorbing-reacting mediumtakes place. The removal of the solute gases so effected may be a purelyphysical phenomenon or may involve solution of the solute gas in aliquid or a slurry suspension, followed by chemical reaction with one ormore constituents in the liquid or the slurry medium, to form a solubleby-product or a solid reaction by-product. The resultant liquid orslurry-laden gas stream is subsequently drawn into a slowly-turning fanthat provides turbulent mixing and additional residence time plus anenvironment for continued absorption and reaction, and for efficientcoalesence or agglomeration of the entrained, sprayed liquid or slurryand its subsequent removal from the system by a simple gravity drain inthe fan casing. An entrainment separator is located downstream from thefan to complete the removal of agglomerated liquid phase (includingslurries) from the system.

The apparatus of the invention may also include means for quenching andcooling a hot gas stream, such as that eminating from electric utilitiesor smelters, with an aqueous solution (water, or other liquids) prior tothe removal of the solute gases.

The apparatus of the invention may also include an effluent hold tankfor closed loop recycling of the absorbing-reacting medium and itsregeneration with fresh make up feed, plus a pumping means to introducethe absorbing-reacting medium into the spray nozzles at the appropriatepressure.

Overall, what has been developed is an improved gas-reacting apparatusin which accelerated absorption and reaction of solute gases in anabsorbing/reacting medium can be effected due to the large surface area,intimate contact, relatively long residence time, and turbulent mixingprevailing therein, thereby overcoming the problem of the prior art, asdiscussed above.

While the invention will be described further, particularly withreference to the removal of solute gases, either by absorption orabsorption accompanied by chemical reactions, it is to be understoodthat the invention is also useful in the conduct of gas-gas reactionsand subsequent wet separation of the resulting reaction product, in theremoval of particulate matter, in the humidification of gases and inreaching a thermal equilibrium between a gas and a liquid.

In a preferred embodiment, the absorption, with or without accompaniedreactions, is conducted in the improved gas reacting apparatus whereinthe unexhausted reacting medium and the reaction products areagglomerated and thereby removed from the gas reacting apparatus as acoherent liquid or slurry mass, depending on the reacting systemselected. In most instances the resulting slurry can be recirculateduntil some optimal concentration is reached, at which point a bleedstream can be removed for further treatment to recover product or forregeneration and recycling purposes, while fresh makeup feed isintroduced into the system prior to recirculation.

In another embodiment, a gas stream containing solute gases and a gasreacting medium are introduced into dual fluid mixing nozzles of thetype described herein having a pair of inlets, one for each incoming gasstream, and a common outlet. The confluence of the two streams in thenozzle creates turbulence which causes the two gas streams to intimatelymix and react substantially instantaneously with each other thereby toproduce reaction products which are in the form of a gas solute orfinely divided solids Wet separation of the above resultant reactionproducts is subsequently carried out as taught by the above preferredcase.

One important feature of the improved gas-reacting apparatus resides inits ability to remove both solute gases and particulate mattersimultaneously with high efficiency, due to the large effectiveinterfacial surface area and the excessively large number of dropletsintroduced to the system, coupled with turbulent mixing and sufficientresidence time that can be effected therein. Still another significantadvantage of the improved gas reacting apparatus, particularly incomparison with the venturis of the prior art, is its ability toaccommodate a very high turndown ratio through a simple adjustment ofthe gas-side pressure drop across the spray nozzles or the amount ofliquid sprayed or both simultaneously. Yet another advantage is anability to provide spray zones of uniform density and, therefore, toyield even gas distribution due to the nozzles being coaxially spacedapart in series within the conduit. The spray zones completely cover thecross sectional area of the conduit and yet without overlapping oneanother, thereby providing good gas and liquid distribution even underupset conditions associated with a nozzle failure. This is preferablyachieved with a unique dual fluid, atomizing spray nozzle design of thetype depicted in the drawings described below that has more precisegas-liquid mixture control and allows for the flexibility required tocontrol size and number of droplets necessary for efficient removal ofsolute gases. The dual fluid spray nozzles generally operate at about 20to about 100 psi, usually at about 20 to about 70 psi, preferably about25 to about 55 psi. The cumulative results of the above-describedadvantages is a gas reacting apparatus which is more economical, moreefficient, more compact and easier to handle than any other moreconventional device. Also, being a relatively small piece of equipment,it can be custom fitted/retrofitted or configured to meet variousspecific site requirements.

These and other characteristic features and advantages of the inventiondisclosed herein will become apparent and more clearly understood fromthe further description given in detail hereinafter with reference tothe attached drawings which form a part thereof.

In one preferred embodiment, a contact chamber is provided located aheadof the scrubber for the removal mainly of particulates from the incominggas stream and is useful, not only in the treatment of gas streamscontaining solute gases which contain particulates but alsoparticulate-contaminated gas streams which do not contain such solutegases.

In such chamber, the entrance and exit are located on opposite sides ofa vertically-located baffle extending normal to the gas flow. Such anarrangement causes the incoming particulate-laden gas stream to impingeon the baffle and then to pass under it to reach the exit. Theperformance of the contact chamber is significantly enhanced by theintroduction of spray nozzles of the type described above for removal ofsolute gases from the gas stream upstream of the fan, one located tospray cocurrently with the gas stream flow and the other located tospray countercurrently to the gas streams.

The nozzles usually are located in the entrance and exit respectively ofthe chamber and impinge on the baffle and preferably are arranged sothat the sprays also substantially fill the inlet and outlet ducts andthe entrance of the gas stream to and the exit of the gas stream fromthe contact chamber. The dynamic action of these fine sprays on theparticulate-laden gas stream combined with the structure of the contactchamber results in removal of significant quantities of particulate fromthe gas stream, often up to 90% or more, regardless of the particlesize.

The gas stream passing from the contact chamber is significantlydepleted with respect to particulate content, enabling very high overallefficiencies, generally in excess of 98%, of particulate removal to beeffected.

The contact chamber also provides the additional residence time oftenrequired to achieve more than about 99% removal of certain acidic gases,notably SO_(x) and NO_(x), from the gas stream via the presence ofsuitable reactants contained in the liquid sprayed into the contactchamber, whether particulate materials are present in the gas stream ornot.

In addition, the contact chamber serves to provide quenching of hot gasstreams to the adiabatic dew point of the gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a gas reacting apparatus and itsaccessories having two separate single spray contact zones constructedin accordance with one embodiment of this invention;

FIG. 2 is an enlarged schematic sectional illustration of a preferreddual-fluid nozzle (Turbotak Caldyn type) used in the apparatus of FIG.1;

FIG. 3 is a schematic view of a portion of the conduit shown in FIG. 1illustraing a cluster nozzle incorporated into one contact spray zoneand includes a sectional view taken on line C--C;

FIG. 4 is a perspective schematic view of the gas reacting apparatus ofFIG. 1;

FIG. 5 is a perspective schematic view of an alternative fan arrangementused with the apparatus of FIG. 1 and having a common exhaust outlet forboth gas and coalesced liquid;

FIG. 6 is a schematic representation of the application of the presentinvention to a typical coal or oil-fired boiler exhaust gas stream forthe removal of gaseous pollutants and fly ash;

FIG. 7 is a perspective, schematic representation of an alternativeembodiment of a gas reacting apparatus of the invention illustrating adouble-loop approach to absorption of solute gases with low reactingmedium; and

FIG. 8 is a schematic representation of a preferred form of a contactchamber for gas quenching and particulate removal, as well as to provideexcellent contact for acidic or other gas removal; and

FIG. 9 is a schematic representation of an alternative form of gascontact chamber for use in environments where space constraints do notpermit normal horizontal flow.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, the gas reacting apparatus 10 shown in FIG. 1comprises in combination an elongated housing 12 defining a primaryreaction zone and a fan 30 defining a post-reaction and coalescencezone.

The elongated housing 12 comprises a relatively straight conduit,preferably of circular cross-section, having an inlet 60 forintroduction of gas stream containing solute gases alone or incombination with particulate matter and an outlet 62 at the other endfor the entry of the resultant liquid or slurry-laden gas stream intothe slowly turning fan 30. The conduit 12 may be positioned in anyorientation with respect to the ground level. However, thegenerally-horizontal position or an orientation in which the conduit ispositioned at a slight angle with respect to the ground level to permitgravitational flow through the conduit, are preferred.

Within the conduit 12 are positioned a number of atomizing spray nozzlemeans 14 for discharging liquid or slurry sprays countercurrent asillustrated (or cocurrent, if desired) to the gas stream flowing throughthe duct 12. The atomizing spray nozzle means 14 provide a very finespray and are capable of delivering droplets in virtually any sizedistribution or quantity required. Typically, the range of about 5 toabout 30 microns liquid droplets is preferred. By spraying liquid orslurry in the above droplet size range into the conduit 12 a tremendousnumber of droplets having a very large interfacial mass transfer area isproduced. For example, if only 5 micron droplets are generated, eachkilogram of liquid will yield about 1.5×10¹³ droplets which have asurface area of about 120 square meters. These figures are orders ofmagnitude greater than generated by any other known contacting device.

In the preferred form as illustrated, liquid or slurry atomizing spraynozzle means 14 comprises dual fluid nozzles each capable of producingthe above droplet size distribution. However, a Turbotak Caldyndual-fluid nozzle (shown in FIG. 2) utilizing gas, i.e. air, steam,etc., to impart the energy required to atomize a liquid is very suitablefor this purpose, particularly when using a slurry as a reacting medium.One of the features of the Turbotak Caldyn nozzle of the type shown inFIG. 2 is that erosion is virtually non-existent. This results becausethe liquid flow or the slurry flow is thought to be contained in anenvelope of gas as it passes through the orifice of the device.

For maximum turbulent mixing and gas-liquid contact time, the scrubbingliquid or slurry preferably should be introduced at a sufficient nozzlepressure and velocity countercurrently to the gas stream to be scrubbedto form the desired spray pattern needed to cover substantially all ofthe cross-sectional area of the interior of the conduit 12 within areasonable distance, e.g. 5 feet from the nozzle. The geometry of thespray issuing from the nozzles 14 and the exact orientation of thenozzles 14 with respect to the conduit 12, apart from being coaxiallyspaced, are not critical. However, for a circular conduit 12, nozzles 14producing a conically-shaped spray pattern, preferably with a sprayangle flow 15° to 90°, is the most advantageous to give adequatecoverage of the conduit cross-section. To effect maximum hold-up ofliquid and gas/liquid contact time, countercurrent flow is used and theatomizing gas pressure preferably should be high enough to impart to theliquid droplets sufficient force to overcome the velocity of theincoming main gas stream, so that no reversal of the sprayed liquid bythe high velocity incoming gas stream can occur until a fully developed,conically-shaped spray pattern, with its extremity touching the conduitwall surface, is established, at which point the sprayed liquid isturned back by the incoming gas stream and becomes suspended therein. Inthis way, if all the energy expended in the sprayed liquid istransferred to the main gas with minimum loss to the conduit wall, avery high degree of turbulence results as the liquid and gas moving inopposite direction come together and the liquid is forced to reversedirection. This high degree of turbulence and increased liquid hold-upand liquid/gas contact time provides extremely efficient contact betweenliquid and gases to yield a very effective and accelerated mass transferof solute gases to the absorbing/reacting medium.

The rate of the flow and pressure of air through the nozzle and thus thedegree of atomization is controlled by a pressure reducing valve 22connected by conduits 20 to the nozzles 14. Gas pressures in the rangeof about 20 to about 60 psi, preferably about 25 to about 50, issupplied to the nozzle by a conduit connected to a gas pressure source23 through a gas regulator (pressure reducing valve) 22. Under suchatomizing pressure conditions and a liquid usage of from about 0.25 to1.0 U.S. gallon per 1000 acf (actual cubic feet) of gas treated, theTurbotak Caldyn dual fluid nozzle has been shown to be capable ofgenerating liquid droplet sizes in the range of about 5 to about 100microns with the majority of droplets having a size of about 5 to 30microns.

For improved mass transfer operation, there may be a number of atomizingspray nozzles 14 employed within the conduit 12. The nozzles 14 arecentrally disposed, countercurrently or cocurrently oriented to the gasflow, axially-spaced apart in series in the conduit and adapted to spraydroplets primarily in the size range from about 5 to 30 microns, therebycreating a number of well back-mixed zones in the conduit 12. Suchorientation of the nozzles results in very high turbulent mixing andhigh interfacial surface area for mass transfer. While, depending on theatomizing pressure employed, the spray nozzles preferably should bepositioned and spaced apart in series in the conduit, so that theconically emerging spray patterns do not substantially overlap eachother. Generally, in the above atomizing pressure range proposed,spacing of approximately four to eight feet was found to be adequate.

It has been found that the use of separated, spaced spray nozzles 14 toprovide at least two separate gas/liquid contact spray zones in whichoppositely moving sprayed liquid and gas come together and the sprayedliquid is forced to reverse direction, provides for removal of typicallyover 99% of the sulfur dioxide and over 99.6% of the particulate matterfrom a synthetic gas stream when scrubbing with aqueous caustic solutionof 0.5 M. This high efficiency is accomplished with the use of about 0.5USG per 1000 acf of gas treated which is only 10% of that required bymost scrubbers with comparable efficiency.

An important feature of the scrubber apparatus of the present inventionresides is its low energy requirement. The approximately 1 to 5 H.P.expended into the liquid per 1000 acf of gas treated and a gas pressuredrop of 0"±W.G. measured across the device are considered to be verylow. To accomplish similar removal effects by mechanically increasingthe gas flow rate by means of blowers pulling through a venturi throatinvolves greater energy coupled with inferior results.

Another significant advantage of the gas reacting apparatus of thepresent invention, particularly in comparison with systems of the priorart, is its ability to accommodate a high turn-down ratio when the flowof the gas stream is decreased because of decreased boiler load withoutthe need for adjustments by moving parts. As can be seen from FIG. 1,the turndown capability of the gas reacting apparatus is not affected bysome mechanical limitation. In the gas reacting apparatus of the presentinvention, the interfacial area is not dependent on the gas flow rate orthe pressure drop. Hence, the solute gas removal efficiency increaseswith reduction in gas flow. One method to regulate make-up feed rate isby controlling the effluent pH. Here a pH electrode probe activates asignal that regulates the position of control valve 53 to control therate of make-up feed through line 55. Other make-up feed control systemmay be used, such as controlling the inlet gas flow and solute gasconcentration or controlling the outlet solute gas concentration as thecontrol variable.

Although the gas reacting apparatus has been described with reference tosingle orifice spray nozzles creating separate spraying zones, it isdesirable and practical in large scale applications where conduits oflarge diameter are used to substitute for the single spray nozzles witha multiple orifice nozzle (i.e. a cluster nozzle) with a combined spraypattern which substantially covers the cross-section of the conduit inorder to obtain maximum effectiveness and space utilization. A schematicview of portion of the conduit illustrating a cluster nozzleincorporated into one contact zone is shown in FIG. 3.

The apparatus 10 of the present invention also comprises a low speed,motor driven fan 30 downstream from the last spray nozzle 14. The fan 30is connected to the outlet of the conduit 12. In particular, fan 30 isof the radial-blade centrifugal type and comprises a shaft 32 having abladed wheel 34 fixed thereto, the shaft and the bladed wheel beingcoaxially positioned or supported in a volute casing 36. In particular,fan wheel 34 (impeller) comprises a disc shaped member 37 fixed to theshaft 32, a plurality of blade members 38 extending from the disc 37 andequally-spaced around the shaft 32, and an annular rim 39 fixed to theedges of blades 38 and disposed in a plane parallel to the plane of thedisc 37. This type of structure, as shown in FIG. 1, was found to beself-cleaning and particularly suitable for severe duty. Other impellertypes, such as the forward curved, backward curved or inclinedstructures, may be used but are not considered to be as suitable as thesimple radial-bladed fan illustrated.

The casing of the fan 30 is formed to include an inlet 40 having aninner diameter smaller than the diameter of annular element 39. Inlet 40is connected to the open end of conduit 12, and an annular joint 41 isprovided to seal the connection. The fan opening preferably should besized to match the size of the conduit or vice versa. However, a taperedinlet 40 or a conduit gradually growing smaller toward the inlet of thefan 30 can also be employed, causing the compressible part of the gasstream to speed up, either at the hub of the fan or in the conduit whilethe incompressible part of the gas stream, i.e. "fly ash" and liquiddroplets, slows down, relative to the velocity of the gas. The increasein the relative velocity between the two phases results in lower gasphase resistance and thus better scavenging action against solute gas byliquid droplets. Also, the larger differences in velocities of theliquid droplets and the gas occur in such tapered inlet caused impactionand results in better scrubbing action against particulates. Anothermethod to improve the turbulent mixing at the inlet to the fan is by theuse of variable guide vanes to impart pre-rotation to the incoming gasstream in an opposite direction to that of the impeller rotation.

The fan drive shaft 32 is connected to the drive means in the form of anelectric motor 42 for rotating shaft 32 and fan wheel 34 at a relativelylow speed. The motor also includes suitable means 43 for maintaining thespeed in a desired range. Fan 30 also includes means in the form of angas exhaust passage from fan 30. Exhaust passage 44 is at the upperportion of casing 36 as viewed in FIGS. 1 and 4 and is connected to oneend of an output duct 46 for exhausting clean gas from the apparatus.The connection of passage 44 to duct 46 is sealed by a joint 47. Fan 30further includes means in the form of an opening 48 provided in volutecasing 36 at the lower end, as viewed in FIGS. 1 and 4, for removing orcollecting sprayed liquid and particulate matter separated from the gasstream. Opening 48 is in fluid flow communication with a relativelyshort conduit or passage 52, disposed generally vertically into a sludgetrap in the form of an open top tank 54. In slurry-based operation, thetank preferably should include an agitator means (not shown in FIG. 1)to keep the solids in suspension. The tank 54 preferably should includeabsorbing/reacting liquid 56 up to a level above the bottom of the tube52. In some cases, it may be desirable to exhaust both the clean gas andthe absorbing/reaction-liquid from the same exhaust passage 44 forsubsequent separation. Such an arrangement is shown in FIG. 5.

The distance between the last spray nozzle 14 and inlet to the fan 30 isnot critical when the nozzle is countercurrently-oriented to the maingas stream flow. However, in applications where the nozzle is directedcocurrent to the gas stream flow, a minimum distance upstream from theintake of the fan 30 generally is required to permit theconically-shaped spray to fully develop and fill completely thecross-section of the conduit 12. In general, a distance of four feet orless has been found to be satisfactory.

The fan 30 also provides means to withdraw and move the gas stream andto overcome the pressure losses across the apparatus. In the fan, thereis provided turbulent mixing and additional residence time plusenvironment for continued absorption and effective coalescence of theliquid droplets and their removal from the system. Much turbulence canalso be effected in countercurrent operation as oppositely moving liquiddroplets suspended in an atomizing gas and the main gas stream cometogether and the liquid droplets are forced to reverse direction and topass through at least two spray contact zones formed by two or morespray nozzles.

The liquid-laden gas is drawn into the vortex of the slowly-turning fan30 in the direction indicated by arrow 62 from which the solutegas-laden droplets and the collected particulate matter exit into thethe liquid or sludge trap provided by opening 48 at the bottom of fan30, as viewed in FIG. 1. Clean gas and some entrained droplets notremoved by the fan, exit through the fan exhaust opening 46. Themajority of the liquid droplets first coalesced in the vortex created bythe fan 30 grow in size and then impinge and constantly coat the fanblades 34, to form a layer of coalesced liquid and solids that adheresto the rotary fan blades and is separated from them mostly on the edgesof the blades by the effects of centrifugal force, moving outwards in sodoing so as to form an attendant annular coarse spray zone to furtherremove solute gas and particulate matter. The liquid droplets adheringto the blades run over the blades, washing them of collected particles.

The particulate matter and solute gas-laden liquid is collected as itreaches the fan housing 36 and draws by gravity into a sump 54 through asealed drain 50. Because the impeller 34 and the fan casing 36 are notcoaxially aligned, the annular space between the impeller and the fancasing increases toward the exhaust opening, thereby preventing anyblockage and interference with the clean gas throughput capacity by thesprayed liquid. Within the sprayed annular zones formed as a result ofcentrifugal force imparted on the liquid coating the blades the main gasstream that has been agitated by the impeller 34 comes into intimatecontact with the reacting liquid discharged from the blades 34, so thatfor all practical purposes, additional removal of solute gases remainingin the gas stream continues to take place.

The flow of clean gas with some entrained liquid droplets not removed bythe fan, continues through the exhaust ducting 46 in the direction ofarrow 64, from which it can be discharged directly to the atmosphere orinto an entrainment separator for final removal of the entrained liquiddroplets. It was found that the fan, upon rotating forward, can separate85 to 90% of the liquid droplets suspended in a gas stream whilebackward rotation can separate some 95% of the suspended liquid.Therefore, for gases containing heavy dust loadings or where a higheroverall removal efficiency is required, a backward turning fan normallyis recommended, although the lower flow rate and static pressure maynecessitate either a larger fan or a booster fan in the system.

The provision of the fan, which constitutes an integral part of the gasreacting apparatus, makes it possible to use the same elements thatserve to move the gas and coalesce the liquid droplets also to providethe turbulent mixing, additional residence time plus mass transfer areasfor continued absorption of solute gases and removal of particulatematter from the gas that is to be treated.

The entrainment separator 70 shown in FIG. 4 to which the clean gas isdischarged from the fan 30 is used to separate entrained liquid notseparated by the fan 30. Typically, a Chevron-type separator which maybe followed by a special mist eliminator packing (Kimre preferred) bothcontained in the same housing were found sufficient to clean the gas ofany suspended liquid. The clean gas then is discharged to the atmospherevia duct 72 at 100% relative humidity, but virtually free of liquidwater content.

A fresh make up of reacting medium feed is added to a recirculation tank54 through line 55. From the tank 54, the scrubbing material is drawnthrough a pump 17 and is introduced into the atomizing spray nozzles 14.Reacting liquid recovered from the clean gas by the fan 30 and theentrainment separator 70 is returned to tank 54 for reuse, while spentscrubbing liquid is discharged through line 57.

Since gas flow in the gas reacting apparatus 10 is unrestricted,pressure drops are low, typically not exceeding 2 inches W.G. Thispressure drop is normally picked up by the fan so that typically, thepressure drop across the system, flange to flange, is zero inches W.G.If desirable, the fan can also pick up system pressure drops up to about6 to 8 inches W.G.

FIG. 5 illustrates an alternate fan arrangement in which a cyclonicseparator 74 is used to effect separation of gas an liquid drawn intothe fan 30.

Referring to FIG. 6, there is illustrated therein the application of thepresent invention to a typical coal- or oil-fired boiler exhaust gas forthe removal of SO₂ therefrom.

As seen, the method lends itself to the use of existing ductwork andI.D. fan, depending on the layout of these in an available plant.Constraints of residence time and temperature of a particularapplication determine whether the existing layout is practical.

As shown in FIG. 6, the gas originating in a flue gas duct 102 from aboiler 100 and exiting gas coolers 104 at temperatures normally rangingup to about 250° C., but not limited to this range, enters a scrubbingarea 106 for simultaneous SO₂ and fly ash removal using the proceduresdescribed above. Adjacent the inlet of the fan 108 a scrubbing medium isinjected countercurrently into the incoming flue gas stream throughinjection 110 to form at least two separate scrubbing zones covering thecross-sectional area of the duct 112 adjacent to the fan 108 whereby theflue gas is scrubbed. After separation of the suspended liquid from thegas by the fan 108 and a downstream entrainment separator 114 andfurther reheating by gas heater 116, a clean gas is discharged to theatmosphere through stack 118. Quenching sprays (not shown in FIG. 6)also may be incorporated where the flue gases are hot to serve to cooland saturate the gas stream with water vapour prior to scrubbing.

Referring now to FIG. 7, there is shown therein a perspective schematicview of a double-loop slurry approach to effect better utilization ofslow reacting solids in suspension such as limestone or iron oxide. Theuse of a double-loop slurry procedure offers greater flexibility becauseextreme operating conditions can be segregated into discrete areas ofthe double loop system, allowing separate chemical and physicalconditions to be maintained. In the double loop slurry procedureillustrated in FIG. 7, a low pH slurry solution contacts the enteringgas stream in an initial reacting loop 200 comprising an elongatedconduit 202 and a plurality of atomizing spray nozzles 204 centrallydisposed and spaced apart from each other in the conduit 202, andadapted to spray slurry into an incoming gas stream whereby some solutegas removal takes place. The slurry-laden gas stream exits from theconduit 202, and enters a hydrocyclone 206 via a tangential inlet 208and swirls down about the vortex finder. The swirling separated slurryconcentrate flows down the cone section to the apex opening 210 which issealed by a joint to the top of a vertically disposed conduit, the otherend of which terminates in a sludge effluent hold tank 212. Theslurry-free, partially-clean gas passes upwards through the vortexfinder to the outlet 214, then to another conduit 216 which is part of asecond reacting loop designed for almost complete removal of theremaining solute gas.

In the second loop, a high pH slurry or solution is contacted with thepartially clean gas in the conduit 216, where the bulk of the solute gasremoval takes place. The second loop comprises an elongated conduit 216,a plurality of spray nozzles 218 coaxially disposed in series to form anumber of reacting zones, a fan 220, an effluent hold tank 222 and anentrainment separator 224. Spent slurry from this loop is discharged tofirst loop via pump 226 and line 228 where the unused reagent isconsumed, thereby proving efficient reagent utilization. Fresh make-upreagent need be added only in the second reacting loop.

This type of design, incorporating two reacting loops in conjunctionwith the gas reacting apparatus of the present invention, takesadvantage of the concept of contacting a gas stream containing thehighest solute gas concentration with the lowest liquor alkalinity in afirst loop to effect good reagent utilization and relatively low solutegas removal and the highest liquor alkalinity with the lowest solute gasconcentration in a second loop to effect poor reagent utilization, butgood solute gas removal. The reduced solute gas removal in the low pHloop (lower alkalinity) is more than offset by improved performance ofthe high-pH loop (higher alkalinity).

Referring now to FIG. 8, there is illustrated therein a preferredcontact chamber 300 which effects an initial treatment of the gas streamand provides a feed to a scrubbing apparatus comprising a single spraynozzle 310 spraying liquid into the gas flowing in the duct 312 to a fan314, operating in the manner described previously. The contact chamber300 is intended to increase turbulence and residence time of the gasstream in a manner superior to the three spray nozzle arrangement ofFIG. 4. This arrangement is of particular significance when a mixture ofacid gas and particulates is to be processed with a high level ofparticulates.

The contact chamber 300 is enlarged in volume in comparison with theduct 312 and comprises a baffle 316 located transversely to the gas flowand a pair of nozzles 318, 320, each arranged to spray liquid at thebaffle 316. The contact chamber 300 is able to remove over 90% of theparticulates contained in the entering gas stream in line and theresulting slurry is conveniently drained, usually continuously, from thelower portion of the chamber 300 by line 322.

It may be necessary to agitate the liquor contained in the lower portionof the chamber to maintain particulates in suspension to facilitateremoval of the slurry, especially if large quantities of particulatesare removed from the contact chamber relative to the amount of liquidused therein.

For removal of fly ash and sulfur dioxide from a coal-fired boiler,water sprays from nozzles 318 and 320 may be used in the contactingchamber 300, which would remove substantial amounts of fly ash but onlyminor quantities of sulfur dioxide. The solids may be separated from theslurry removed by line 322 by thickening and/or filtration andthereafter sent to landfill. The aqueous phase from such separation,which is acidic from the dissolved SO₂, may be recycled with make-up tothe contacting chamber nozzles or may be made basic and used as make-upliquor for the SO₂ removal stage at the nozzle 310. The nozzle 310 isfed with a basic aqueous solution to remove the gaseous SO₂ in the duct312 downstream of the contacting chamber 300.

Alternatively, a basic solution may be fed to the nozzles 318, 320,which has the effect of removal of larger quantities of SO₂ in thechamber 300, so that lesser quantities are required to be removed in theduct 312. If longer residence times are required, a second contactchamber may be used and thereby enhance SO₂ removal.

It may be desirable under some circumstances to employ an entrainmentseparator between the contact chamber 300 and the nozzle 310 to assistin maintaining the specific conditions conducive to each stage.

Any particulate material remaining in the gas stream following thecontact chamber 300 and entering the scrubber section at nozzle 310 isremoved from the gas stream along with the SO₂. Thickening or filtrationof the resulting scrubbing liquor separates out the solids. For the casewhere a water-soluble scrubbing agent is used, for example, sodiumhydroxide or sodium sulfite, the filtered solution may be contacted witha hydrated lime slurry in a conventional dual alkali process with thebasic sodium sulfite being returned to the nozzle 310.

An alternative arrangement is shown in FIG. 9, in which the inlet pipe321 is in a vertically-downward orientation, with the nozzle 318 againlocated in the entrance to the chamber 310', where space constraints donot permit the normal horizontal flow. An optional additional nozzle 324may be provided on the downstream side of the baffle 316 for additionalscrubbing, as required, both in chamber 310 and 310'.

EXAMPLES

The following specific Examples illustrate the use of the gas reactingapparatus of the present invention for the purpose of removing SO₂ fromsynthetic gases by NaOH and NH₃ aqueous solutions and a lime slurrycontaining MgO.

EXAMPLE I

This Example illustrates the use of the gas reacting apparatus of FIG. 1for the purpose of removing SO₂ from a synthetic gas stream containingabout 1100 ppm SO₂, 21% V O₂ and the balance nitrogen, by absorptioninto aqueous NaOH solution of sufficient concentration of active sodiumalkalinity.

In this type of removal, absorption accompanied by chemical reactiontakes place between the SO₂ and NaOH to form soluble sodium-basedsulfite, bisulfite and sulfate compounds, which effectively traps SO₂ inthe solution. With the caustic system having an initial active sodiumconcentration of 0.3 M (pH 12.4), a liquid- to-gas ratio of 1.0 USG per1000 acf of gas treated and a ratio of active molar concentration ofsodium to moles of SO₂ inlet of 1.2:1, 99% SO₂ removal was effected. Toeffect the same degree of SO₂ removal but at a lower pH of 6.2, a L/Gratio of 4.75:1 was required.

When the concentration of the aqueous NaOH solution was increased to 0.5M active Na (pH 12.5), a liquid-to-gas ratio as low as 0.5 USG per 1000acf of gas treated was required to operate the reactor to effect 99% SO₂removal. These results were obtained by using three spray nozzles inseries to form three separate reacting zones within a single conduit.The nozzles were oriented countercurrently to the gas stream flow. Thepressure drop across the conduit was less than 2 inches W.G. duringthese tests.

EXAMPLE II

This Example further illustrates the use of the gas reacting apparatusof FIG. 1 for the removal of SO₂ from synthetic gas stream containingabout 1100 ppm SO₂, 21% V O₂ and the balance nitrogen, by scrubbing withan ammoniacal solution.

The SO₂ removal efficiency averaged well above 95% which was maintainedat this level as long as the NH₃ -to-SO₂ feed stoichiometry was higherthan 1.9:1. With an NH₃ -to-SO₂ feed stoichiometry of from 1.8 to 2.0:1,the effect of liquid flow rate on SO₂ removal over the range of liquidrates of 0.005 to 0.5 USGM (corresponding to a L/G ratio range from 0.17to 1.15) was observed to be minor and in general high removalefficiencies were obtained ranging from 95 to 99%.

In these tests, the ammonia gas feed to the system was introduced withthe atomizing gas (air) into three pneumatic, dual-fluid nozzlescoaxially disposed in series in a conduit and using recycled scrubbingliquor as the liquid phase. It was observed that the high turbulence,swirling and pressure conditions prevailing at the nozzles enhancedsubstantially the chemisorption of the ammonia in the sprayed liquidphase.

In this method of ammonia injection, there was also evidence ofsubstantial suppression of a plume (commonly associated with ammoniascrubbing operations) exiting the apparatus in all of the tests soconducted and it may have been due to the manner in which the gaseousammonia was admitted to the system.

The Table below shows the SO₂ removal efficiency obtained as a functionof the reactor outlet pH and NH₃ /SO₂ stoichiometry employed at an L/Gof about 1.0 USG per 1000 acf of gas treated.

                  TABLE                                                           ______________________________________                                        NH.sub.3 /SO.sub.2 Stoichiometry,                                                           Reactor pH SO.sub.2 Removal Efficiecy                           ______________________________________                                        1.09          3.9        55                                                   1.24          5.4        81                                                   1.52          6.6        92                                                   1.89          7.4        95                                                   2.00          8.5        99                                                   ______________________________________                                    

EXAMPLE III

This Example further illustrates the use of the gas reacting apparatusof FIG. 1 for removal of SO₂ from a synthetic gas stream using a limeslurry containing MgO. In this case a dolime assaying 35.9 wt % Ca and20 wt % Mg in the form of a finely divided powder, was slaked to give aslurry of some 1.9 wt % solids loading.

A synthetic gas stream containing about 1200 to 1400 ppm SO and 21% Voxygen was produced at the rate of 550 to 650 acfm by adding SO₂ gasfrom cylinders to the inlet air stream. The temperature of the gas wasambient.

The system was operated in a recirculating mode during which continuousaddition of make up dolime slurry was added at the rate of 0.32 lb/minfor 145 minutes to provide for the required stoichiometric amount ofalkalinity and to maintain the recycled tank pH at a prescribed level ofbetween 6.0 and 7.0. Under such pH conditions, it was found that a highconcentration of dissolved alkalinity (present as magnesium sulfite) inthe reacting liquor occurred, resulting not only in a well-bufferedreacting solution but also in a scale-free operation of highreliability.

SO₂ removal of 95 to 97% was achieved with the gas reacting apparatus atgas-to-liquid ratio of 4.5 gal/10³ acf. This scrubbing efficiencyremained close to the above values for the duration of the test.

Operating experience with the gas reacting apparatus of FIG. 1 usingdifferent commercially available reacting agents has shown that, formost of the systems studied, under optimal pH conditions and reagentconcentration, an L/G of only 1 to 5 USG per 1000 acf of gas treatedappears to be adequate to maintain uniformly and consistently high SO₂removal. For reactive systems, such as the sodium and ammonia-basedsystems, the apparatus provides excellent SO₂ gas removal in excess of99% and, if necessary, also efficient simultaneous removal ofparticulate matter in excess of 99.6%, and yet permitting aliquid-to-gas ratio in the range from 0.17 to 0.5 USG per 1000 acf ofgas treated.

This low L/G ratio requirement employed by the gas reacting apparatus ofthe invention should not only reduce both capital and operating costs toa fraction of the costs related to traditional removal devices, butshould also enable easy integration into flue gas ductwork of existingoil or coal-fired boilers due to its compact size. As shown particularlyin FIG. 6, the apparatus of the invention can be configured to easilymeet site requirements.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention relates to thegas/liquid, gas/liquid/solid and gas/gas/liquid mass transfer art andmore particularly to an improved method and gas reacting apparatus forwet mass transferring of solute gases from process gas streams into aliquid or slurry reacting medium, wherein the mass transfer operationmay be a purely physical phenomenon or may involve solution of thematerial in the absorbing liquid or slurry, followed by reaction withone or more constituents in the absorbing liquid or slurry medium.

The improvement provides an apparatus in which accelerated absorptionand reaction of solute gases can be effected as a result of the largeinterfacial surface area for mass transfer, plurality of reaction zones,intimate contact, increased residence time and turbulent mixingprevailing therein.

While an improved apparatus and method have been described in detail,various modifications, alterations and changes may be made withoutdeparting from the spirit and scope of the present invention as definedby the appended claims.

What we claim is:
 1. A gas reacting apparatus for the wet mass transferof solute gases from a process gas stream into a reaction medium,comprising:(a) an elongated conduit means having an inlet end and anoutlet end to permit a process gas stream to enter through said inletend, flow along said conduit means and pass out through said outlet end;(b) plurality of dual-fluid spray means spaced in series within saidconduit means for the formation of a plurality of spray contact zones ofuniformly-distributed fine spray having a droplet size of from about 5to about 100 microns of a reacting medium wherein intimate contactbetween the reacting medium and the process gas stream is effected toremove solute gases from the process gas stream; and (c) fan meansconnected in fluid flow communication with said outlet end of saidconduit means for coalescing liquid droplets in the gas stream passingto said outlet end of said conduit means and including a drive means foroperating said fan means at a relatively low speed to effect thecoalescence, means defining a gas exhaust passage for clean gasseparated by said fan means and means defining a passage for dischargingliquid or slurry separated from the gas stream by said fan means as aresult of the coalescence.
 2. The apparatus of claim 1 wherein saidplurality of spray means are coaxially arranged.
 3. The apparatus ofclaim 2 wherein at least one of said plurality of spray means isarranged to be countercurrently directed with respect to the directionof flow of the gas stream from said inlet end to said outlet end.
 4. Theapparatus of claim 3, operating in a once through mode and furtherincluding:(d) means for supplying gas under pressure to said pluralityof dual fluid spray means; (e) means for introducing a reacting mediumas a liquid solution or a slurry suspension into said plurality of dualfluid spray means under pressure; (f) tank means for holding liquid orslurry discharged from said fan means; and (g) means for separatingentrained liquid or slurry not separated by said fan means from a gasstream.
 5. The apparatus of claim 3 operating in a closed loop mode andfurther including:(h) means for supplying gas under pressure to saidplurality of dual fluid spray means; (i) tank means for holding liquidor slurry discharged from said fan means; (j) means for adding freshmake-up feed to said tank means; (k) means for blowing down spent liquidor slurry from said tank means; (l) means for recycling the liquid orslurry to said dual-fluid spray means to provide a reacting medium; and(m) means for separating entrained liquid or slurry not separated bysaid fan means from a gas stream.
 6. The apparatus of claim 1 whereinsaid conduit means has a circular cross-section.
 7. The apparatus ofclaim 1 wherein said conduit means is relatively straight and isgenerally horizontally oriented.
 8. The apparatus of claim 1 wherein allof said plurality of dual-fluid spray means are directedcountercurrently to the gas stream.
 9. The apparatus of claim 1 whereinall of said plurality of dual-fluid spray means are directedconcurrently to said gas stream.
 10. The apparatus of claim 1 whereineach spray contact zone is formed by a single, dual fluid spray means.11. The apparatus of claim 1 wherein each spray contact zone is formedby a plurality of dual fluid spray means integrated into one body. 12.The apparatus of claim 1 wherein the dual fluid spray means arecoaxially separated from each other by a distance sufficient to enablethe spray pattern to fill the conduit means with an extremity thereoftouching the walls of the conduit means and yet without substantiallyoverlapping each other.
 13. The apparatus of claim 1 wherein said fanmeans is of the radial-blade centrifugal type having a bladed fan wheelrotatably mounted in a volute casing and having a clean gas exhaustopening portion in said casing and a bottom opening drain in said casingfor removing separated liquid or slurry therefrom.
 14. The apparatus ofclaim 1 wherein said volute casing has means defining a common dischargeopening for both the clean gas and liquid adjacent to the lower portionof said casing.
 15. The apparatus of claim 1 wherein the fan wheelcomprises a disc-shaped member fixed to the drive shaft, a plurality ofblade members vertically extending from said disc-shaped member andequally spaced from each other.
 16. The apparatus of claim 1 wherein theaxis of rotation of said fan means is generally coincident with the axisof said conduit means.
 17. The apparatus of claim 1 wherein said fanmeans includes a drive for forward or backward rotation.
 18. Theapparatus of claim 1 including gas contact chamber means connected tothe inlet end of said elongated conduit means, said chamber meanscomprising:an enclosure having an inlet for a gas stream, an outlet fora gas stream and a gas flow path between said inlet and said outlet, abaffle extending vertically downwardly from an upper closure to andwithin said enclosure and located normal to the gas flow path, wherebythe gas stream contacts said baffle within said chamber and then passesunder said baffle at a relatively lower velocity to pass from said inletto said outlet, and a plurality of dual-fluid spray means one beinglocated co-current to the gas flow path on an inlet side of said baffleand one being located counter-current to the gas flow path on an outletside of said baffle, each of said dual-fluid spray means beingconstructed to form a uniformly-distributed fine spray of contact mediumwhich impinges on said baffle.
 19. A gas contact chamber apparatus,comprising:an enclosure having an inlet for a gas stream located in anupper portion of said enclosure, an outlet for a gas stream also locatedin an upper portion of said enclosure and a gas flow path between saidinlet and said outlet, a baffle extending vertically downwardly from anupper closure to and within said enclosure and normal to the gas flowpath for a sufficient distance to cause the gas stream entering saidenclosure through said inlet to contact an inlet side of said baffle andthen to pass under said baffle at a relatively lower velocity to saidoutlet, and at least two dual-fluid spray means one being locatedadjacent said inlet and co-current to the gas flow path to form auniformly-distributed fine spray of contact medium impinging on theinlet side of said baffle, and one being located adjacent said outletand countercurrent to the gas flow path to form a uniformly-distributedfine spray of contact medium impinging on the outlet side of saidbaffle.
 20. The apparatus of claim 19 wherein said inlet comprises asubstantially horizontally-extending inlet pipe, said one spray meansadjacent said inlet is located substantially axially in said inlet pipe,said outlet comprises a substantially horizontally-extending outlet pipeand said one spray means adjacent said outlet is located substantiallyaxially in said outlet pipe.
 21. The apparatus of claim 20 wherein saidspray means are respectively arranged in said inlet and outlet pipe suchthat sprays produced thereby substantially fill the inlet and outletrespectively.
 22. The apparatus of claim 19 wherein said inlet comprisesa substantially vertically-extending inlet pipe, said one spray meansadjacent said inlet is located substantially axially in said inlet pipe,said outlet comprises a substantially horizontally-extending outlet pipeand said one spray means adjacent said outlet is located substantiallyaxially in said outlet pipe.
 23. An apparatus for removal of materialfrom a gas stream, comprising:a gas contact chamber apparatus as definedin claim 19; an elongate conduit means having an inlet end communicatingwith said outlet of said gas contact chamber apparatus and an outletend, to permit a gas stream to enter from said gas contact chamberapparatus through said inlet end, flow along said conduit means and passout through said outlet end, at least one dual-fluid spray means locatedin said conduit means for the formation of at least one spray contactzone of uniformly-distributed fine spray of an aqueous contact mediumwherein intimate contact between the sprayed medium and the gas streamis effected to remove materials from the gas stream; and fan meansconnected in fluid flow communication with said outlet end of saidconduit means for coalescing liquid droplets in the gas stream passingto said outlet end of said conduit means and including a drive means foroperating said fan means at a relatively low speed to effect thecoalescence, means defining a gas exhaust passage for clean gasseparated by said fan means and means defining a passage for dischargingslurry separated from the gas stream by said fan means as a result ofcoalescence.